Methods for manufacturing a semiconductor device having a non-ohmic contact formed between a semiconductor material and an electrically conductive contact layer

ABSTRACT

An embodiment of a method of manufacturing a semiconductor device includes providing a semiconductor material that comprises SiC and forming an electrically conductive contact layer on the semiconductor material. A non-ohmic contact is formed between the semiconductor material and the electrically conductive contact layer. The electrically conductive contact layer comprises a metal nitride with a nitrogen content between 10 to 50 atomic %. Additional embodiments of manufacturing a semiconductor device are described.

BACKGROUND

Schottky diodes comprising a metal-semiconductor junction are generallyused as rectifying devices. In particular, SiC Schottky diodes areincreasingly used in the field of power electronics.

It is an object of the present invention to provide an improvedsemiconductor device comprising a metal-semiconductor junction. Further,it is an object to provide a method of manufacturing such asemiconductor device.

SUMMARY

According to an embodiment, a semiconductor device comprises asemiconductor material having a bandgap larger than 2 eV and less than10 eV, and a contact layer in contact with the semi conductor material,the contact layer comprising a metal nitride. A non-ohmic contact isformed between the semiconductor material and the contact layer.

According to a further embodiment, a semiconductor device comprises asemiconductor body including a semiconductor material having a bandgaplarger than 2 eV and less than 10 eV, and a contact layer in contactwith a first surface of the semiconductor body. The contact layercomprises a metal nitride. The contact layer is electrically connectedto a first load terminal, and a non-ohmic contact is formed between thesemiconductor body and the contact layer. A second surface of thesemiconductor body is electrically connected to a second load terminal,the second surface being opposite to the first surface.

According to a further embodiment, a semiconductor device comprises asemiconductor body including a semiconductor material having a bandgaplarger than 2 eV and less than 10 eV, and a contact layer in contactwith a first surface of the semiconductor body. The contact layercomprises a metal nitride. The contact layer is electrically connectedto a first load terminal. A non-ohmic contact is formed between thesemiconductor body and the contact layer. A second surface of thesemiconductor body is electrically connected to a second load terminal,the second surface being opposite to the first surface.

Those skilled in the art will recognize additional features andadvantages upon reading the following detailed description, and uponviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of embodiments of the invention and are incorporated inand constitute a part of this specification. The drawings illustrate theembodiments of the present invention and together with the descriptionserve to explain the principles. Other embodiments of the invention andmany of the intended advantages will be readily appreciated, as theybecome better understood by reference to the following detaileddescription. The elements of the drawings are not necessarily to scalerelative to each other. Like reference numbers designate correspondingsimilar parts.

FIG. 1A illustrates a cross-sectional view of an example of asemiconductor device according to an embodiment.

FIG. 1B shows a cross-sectional view of a semiconductor device accordingto a further embodiment.

FIG. 1C shows a cross-sectional view of a semiconductor device accordingto a further embodiment.

FIG. 1D shows a cross-sectional view of a semiconductor device accordingto a further embodiment.

FIG. 2A illustrates an example of a current-voltage characteristic of anohmic contact.

FIG. 2B illustrates a cross-sectional view of a rectifying contact.

FIG. 3 illustrates an energy band diagram of a Schottky contact.

FIG. 4 illustrates an example of a reaction chamber which may be usedfor manufacturing the semiconductor device according to an embodiment.

DETAILED DESCRIPTION

In the following detailed description reference is made to theaccompanying drawings, which form a part hereof and in which areillustrated by way of illustration specific embodiments in which theinvention may be practiced. In this regard, directional terminology suchas “top”, “bottom”, “front”, “back”, “leading”, “trailing” etc. is usedwith reference to the orientation of the Figures being described. Sincecomponents of embodiments of the invention can be positioned in a numberof different orientations, the directional terminology is used forpurposes of illustration and is in no way limiting. It is to beunderstood that other embodiments may be utilized and structural orlogical changes may be made without departing from the scope defined bythe claims.

The description of the embodiments is not limiting. In particular,elements of the embodiments described hereinafter may be combined withelements of different embodiments.

The terms “wafer”, “substrate” or “semiconductor substrate” used in thefollowing description may include any semiconductor-based structure thathas a semiconductor surface. Wafer and structure are to be understood toinclude silicon, silicon-on-insulator (SOI), silicon-oil sapphire (SOS),doped and undoped semiconductors, epitaxial layers of silicon supportedby a base semiconductor foundation, and other semiconductor structures.The semiconductor need not be silicon-based. The semiconductor could aswell be silicon-germanium, germanium, or gallium arsenide. According toother embodiments, diamond, silicon carbide (SiC) or gallium nitride(GaN) may form the semiconductor substrate material.

The Figures and the description illustrate relative dopingconcentrations by indicating “−” or “+” next to the doping type “n” or“p”. For example, “n⁻” means a doping concentration which is lower thanthe doping concentration of an “n”-doping region while an “n⁺”-dopingregion has a higher doping concentration than an “n”-doping region.Doping regions of the same relative doping concentration do notnecessarily have the same absolute doping concentration. For example,two different “n”-doping regions may have the same or different absolutedoping concentrations. In the Figures and the description, for the sakeof a better comprehension, often the doped portions are designated asbeing “p” or “n”-doped. As is clearly to be understood, this designationis by no means intended to be limiting. The doping type can be arbitraryas long as the described functionality is achieved. Further, in allembodiments, the doping types can be reversed.

As employed in this specification, the terms “coupled” and/or“electrically coupled” are not meant to mean that the elements must bedirectly coupled together—intervening elements may be provided betweenthe “coupled” or “electrically coupled” elements. The term “electricallyconnected” intends to describe a low-resistive electric connectionbetween the elements electrically connected together.

As used herein, the terms “having”, “containing”, “including”,“comprising” and the like are open ended terms that indicate thepresence of stated elements or features, but do not preclude additionalelements or features. The articles “a”, “an” and “the” are intended toinclude the plural as well as the singular, unless the context clearlyindicates otherwise.

The terms “lateral” and “horizontal” as used in this specificationintends to describe an orientation parallel to a first surface of asemiconductor substrate or semiconductor body. This can be for instancethe surface of a wafer or a die.

The term “vertical” as used in this specification intends to describe anorientation which is arranged perpendicular to the first surface of thesemiconductor substrate or semiconductor body.

FIG. 1A shows a cross-sectional view of a semiconductor device 1according to an embodiment. The semiconductor device 1 illustrated inFIG. 1A comprises a semiconductor material 100 and a contact layer 130in contact with the semiconductor material 100. As will be explained inthe following, the contact layer 130 comprises a metal nitride and anon-ohmic contact is formed between the semiconductor material 100 andthe contact layer 130. A bandgap of the semiconductor material 100 islarger than 2 eV and less than 10 eV, e.g. less than 6 eV. According toa further embodiment, the bandgap of the semiconductor material may belarger than 0.9 or 1 eV and less than 10 eV.

The semiconductor material 100 may be a semiconductor body 101 includingone or more doped portions or layers at either side thereof. The dopedportion may be formed by various methods such as ion implantation,diffusion and epitaxial growth of the doped layer. For example, thesemiconductor material may be a material having a bandgap larger than 1eV. By way of example, the semiconductor material may comprise siliconcarbide, diamond, gallium nitride, indium phosphide, AlGaAs and furtherexamples of III-V semiconductors.

For example, the semiconductor body 101 may be heavily n-doped and maycomprise a portion that is n-doped at a lower doping level, the portionbeing disposed at a first surface 110 of the semiconductor body. Thecontact layer 130 may comprise a combination of a stoichiometriccompound with a non-stoichiometric compound including the metal andnitrogen. For example, the contact layer 130 may comprise mixture ofM_(x)N_(y) having different values for x and y, wherein M denotes themetal. For example, the metal nitride layer may comprise a mixture of MNand M_(x)N_(y) or of MN₂ and M_(x)N_(y). Generally, in these formulas, xmay be equal to 1, and y may be a real number fulfilling 0<y<3.Alternatively, y may be equal to 1, and x may be a real numberfulfilling 0<x<3. For example, the metal may be selected from the groupof molybdenum, titanium, tantalum, and tungsten. Further, the metalnitride may comprise two metals, such as MoTiN.

The contact layer 130 may be electrically connected to an anodeterminal. Further, the semiconductor device 1 may comprise a back sidemetallization 160 which forms an ohmic contact to the semiconductor body101. The back side metallization 160 is disposed at a second surface 115of the semiconductor body 101, opposite to the first surface 110. Theback side metallization 160 may be electrically connected to a cathodeterminal. The terms “ohmic contact”, “Schottky contact”, and “rectifyingcontact” will be explained below, while referring to FIGS. 2A and 2B.

FIG. 1B shows a further embodiment of a semiconductor device 1. Thesemiconductor device 1 illustrated in FIG. 1B comprises a semiconductormaterial 200, e.g. a semiconductor body 201 and a contact layer 130 incontact with the semiconductor material. The contact layer 130 comprisesa metal nitride and a non-ohmic contact is formed between thesemiconductor material 200 and the contact layer 130. Generally, thesemiconductor material 200 may be a semiconductor body 201 includingfurther doped portions. For example, these doped portions may bedisposed adjacent to a first surface 210 or a second surface 215 of thesemiconductor body 201. Differing from the embodiment of FIG. 1A, thesemiconductor device 1 of FIG. 1B further comprises a doped region 180of the second conductivity type. For example, the semiconductor material200 may be n⁻-doped and the doped portion 180 may be p⁺-doped. The dopedportion 180 may be disposed at the first surface 210 of thesemiconductor body 201, and portions of the semiconductor material 200,which is e.g. n⁻-doped may be present at the first surface 210. Thecontact layer 130 may be in contact with the semiconductor material 200and the doped portions 180. The semiconductor device 1 illustrated inFIG. 1B further comprises a heavily n⁺-doped region 170 at the secondsurface of the semiconductor body 201. The semiconductor device furthercomprises a back side metallization layer 160 that forms an ohmiccontact to the doped layer 170. The back side metallization layer 160may be electrically connected to a cathode terminal. The contact layer130 may be electrically connected to an anode terminal. Thesemiconductor material may comprise any of the materials mentionedabove. For example, the semiconductor material may be silicon carbide.

The semiconductor device illustrated in FIG. 1B may implement aJunction-Barrier Schottky (JBS) diode including p⁺ implanted portions180. When a reverse voltage is applied to the semiconductor device,depletion regions formed at the interface between the n⁻ portion 200 andthe p⁺ portion 180 pinch off a leakage current which may arise from theSchottky contact of the device. Accordingly, such a Junction-BarrierSchottky diode has a reduced leakage current. Such a JBS may be suitablyused in a switched mode power supply.

According to a further embodiment, the semiconductor device 1 mayimplement a merged PIN Schottky diode (MPS). FIG. 1C shows across-sectional view of such a merged PIN Schottky-Diode. The MPScomprises similar components as the JBS, these components having thesame reference numerals as the corresponding components of the JBS. Inparticular, the p⁺ portions 185 of the MPS are configured to injectminority carriers into the n⁻ portion 200 in a forward direction. Forexample the p⁺ portions 185 may be doped at a high doping concentration,e.g. 10¹⁹ to 10²⁰ cm⁻³.

FIG. 1D illustrates a further example of a semiconductor device. As isshown, the semiconductor device 1 comprises a semiconductor body 101comprising a semiconductor material having a band gap larger than 2 eVand less than 10 eV, a contact layer 130 in contact with a first surface110 of the semiconductor body 101, the contact layer 130 comprising ametal nitride. The contact layer 130 is electrically connected to afirst load terminal 240. A non-ohmic contact is formed between thesemiconductor body 101 and the contact layer 130. A second surface 115of the semiconductor body 101 is electrically connected to a second loadterminal 250. The second surface 115 is opposite to the first surface110.

For example, the contact layer 130 may be in contact with the dopedportion 120. According to an embodiment, the semiconductor body 101 maybe heavily n⁺-doped and may be of an n-conductivity type. The dopedportion 120 may be of an n-conductivity type, at a lower concentrationof the n-type dopants.

For example, the semiconductor device shown in FIG. 1D may implement aSchottky-diode or a Schottky-diode related device. In this case, thefirst load terminal 240 may be an anode terminal, and the second loadterminal 250 may be a cathode terminal. Depending from differentimplementations of the semiconductor device, the first load terminal 240may be a source terminal and the second load terminal 250 may be a drainterminal, e.g. in the case of a MOSFET (metal oxide semiconductor fieldeffect transistor) or a JFET (junction field effect transistor).According to a further example, the first load terminal 240 may be anemitter terminal, and the second load terminal 250 may be a collectorterminal, e.g. in the case of an IGBT (insulated gate bipolartransistor).

The semiconductor device may comprise an active region 181 and ajunction terminal area 182. In the active region 181, the contact layer130 is in contact with the semiconductor body 101. The junction terminalarea 182 is different from the active region 181 with regard to functionand structure. To be more specific, in the active region 181, a loadterminal of the semiconductor device, e.g. the anode terminal iselectrically connected to the semiconductor body for the purpose ofcurrent conduction. In contrast, the purpose of the junction terminationarea is edge termination for reducing the electric field peak at theperiphery of the semiconductor device 1. Typical structural elements ofthe junction termination area include one or more of field plates, ringstructures such as floating guard rings or ring segments, junctiontermination extension (JTE) structures and variation of lateral doping(VLD) structures, for example.

FIG. 2A shows an example of a current-voltage characteristic of an ohmiccontact. As can be seen, the current is approximately proportional withrespect to the applied voltage. The ratio of voltage and current isdenoted as the resistance of the contact.

On the other hand, as is illustrated in FIG. 2B, across a non-ohmiccontact the current need not be proportional with respect to thevoltage. Rather, as can be seen on the left-hand side of the chartillustrated in FIG. 2B, almost no current may be flowing, independentfrom the negative voltage applied. Further, when applying a positivevoltage, the current may increase in a non-linear manner. Any kind ofcurrent-voltage characteristics in which the current is non-linear tothe applied voltage, may be regarded as establishing a non-ohmiccontact. For example, the contact may be a rectifying contact such as,for example, a pn junction or a Schottky junction, in which only a smallcurrent, i.e. the reverse saturation current flows, when a low voltagein a reverse direction is applied. When a higher voltage is applied inthe reverse direction, a breakdown current may flow.

In the context of the present specification, the term “non-ohmiccontact” is understood to represent any kind of contact having anon-linear current-voltage characteristics. According to a furthermodification, the term “rectifying contact” is considered to representany kind of contact according to which only a little or no currentflows, when a voltage in a reverse direction is applied, the current notbeing proportional with respect to the applied voltage.

FIG. 3 shows an example of an energy band diagram of a rectifying metalsemiconductor junction. The right-hand side of FIG. 3 shows the energyband diagram within the semiconductor material, wherein W_(C) denotesthe energy level of the conduction band, W_(V) denotes the energy levelof the valence band and W_(F) denotes the Fermi level of thesemiconductor material. The difference ΔW between the energy level W_(C)of the conduction band and the energy level W_(V) of the valence banddenotes the bandgap of the semiconductor material. The left-hand portionof the energy band diagram of FIG. 3 shows the work function q×φ_(M) ofthe metal. When the metal and the semiconductor material form ajunction, a potential barrier is generated at the interface between theFermi level of the metal W_(F) and the valence band of the semiconductormaterial. The height of the potential barrier q×φ_(B) also is referredto as the “Schottky barrier” of the contact.

Generally, Schottky contacts including a semiconductor material having awide bandgap have a large forward voltage drop due to the work functionand the Schottky barrier of the contact metals used. According to thedescribed embodiment, by selecting a contact layer including a metalnitride, the height of the Schottky barrier may be adjusted. Inparticular, by varying the nitrogen content of the metal nitride, thework function of the metal may be suitably set. As a consequence, theSchottky barrier and hence, the forward voltage drop may be set bysetting the nitrogen content of the metal nitride. For example, thenitrogen content of the metal nitride may be more than 10 atomic % andless than 50 atomic %, more specifically, from 38 to 45 atomic %. Forexample, the nitrogen content may be determined using Auger ionspectroscopy, secondary ion mass spectroscopy (SIMS) or X-RayPhotoelectron Spectroscopy (XPS).

The semiconductor device may be a semiconductor component which may beselected from the group consisting of a Schottky diode, a merged pnSchottky diode, a JFET, a MESFET, an integrated flyback diode, arectifier, an inverter and a power supply.

FIG. 4 illustrates a reaction chamber of a sputter processing apparatusin which the contact layer comprising a metal nitride may be formed. Asemiconductor substrate 430 may be disposed on a rotatable table 440. Asputter target 410 may be attached to a supporting element 415. Thetarget may comprise the metal that forms the metal nitride. For example,the target may be made of a metal selected from the group consisting ofmolybdenum, titanium, tantalum, and tungsten. Further, the target 410may comprise a combination of these metals. The chamber 400 comprises agas inlet 420 through which a plasma forming inert gas such as argon maybe fed into the reaction chamber. Further, nitrogen (N₂) may be fed viathe inlet 420. After igniting a plasma nitrogen is a reactive gas whichreacts with the atoms of the target 410. An electrical field as well asa magnetic field may be applied to the sputtering apparatus. Furtherdetails of the sputtering method are generally known.

According to an embodiment, by setting the partial pressure of nitrogen,the content of the nitrogen in the deposited metal nitride layer may bedetermined. It has been shown, that thereby the work function of thecontact layer may be changed. For example, the barrier height of aMo_(x)N_(y) metal in contact with a silicon carbide layer may be 0.94 eVto 1.12 eV. For example, the total pressure within the sputteringchamber may be 4 to 15 mTorr. The partial pressure of nitrogen(N₂/(N₂+Ar)) may be 0.1 to 1.0.

While embodiments of the invention have been described above, it isobvious that further embodiments may be implemented. For example,further embodiments may comprise any subcombination of features recitedin the claims or any subcombination of elements described in theexamples given above. This application is intended to cover anyadaptations or variations of the specific embodiments discussed herein.Therefore, it is intended that this invention be limited only by theclaims and the equivalents thereof.

What is claimed is:
 1. A method of manufacturing a semiconductor device,the method comprising: providing a semiconductor material that comprisesSiC; and forming an electrically conductive contact layer on thesemiconductor material, wherein a non-ohmic contact is formed betweenthe semiconductor material and the electrically conductive contactlayer, wherein the electrically conductive contact layer comprises ametal nitride with a nitrogen content between 10 to 50 atomic %.
 2. Themethod of claim 1, wherein the electrically conductive contact layer isformed by a reactive sputtering process using nitrogen as a reactant. 3.The method of claim 2, wherein a partial pressure of nitrogen during thereactive sputtering process is set so as to form the metal nitride as amixed phase of MN and M₂N, and wherein M denotes the metal.
 4. Themethod of claim 3, wherein a ratio of the partial pressure of nitrogenand a total pressure during the reactive sputtering process is in arange of 0.1 to 1.0.
 5. The method of claim 3, wherein the partialpressure of nitrogen during the reactive sputtering process is set so asto set a work function of the electrically conductive contact layer. 6.The method of claim 1, wherein the metal nitride comprises a metalselected from the group consisting of molybdenum, titanium, tantalum andtungsten.
 7. The method of claim 1, wherein the metal nitride comprisestwo metals.
 8. The method of claim 1, wherein the metal nitridecomprises molybdenum.
 9. The method of claim 1, wherein the metalnitride comprises a combination of a stoichiometric compound and anon-stoichiometric compound including a metal and nitrogen.
 10. Themethod of claim 1, wherein the metal nitride layer comprises a mixtureof M_(x)N_(y) and at least one of MN and MN₂, and wherein either x=1 and0<y<3 or y=1 and 0<x<3.
 11. The method of claim 1, wherein the non-ohmiccontact is a rectifying contact.
 12. The method of claim 1, wherein thenon-ohmic contact is a Schottky contact.
 13. The method of claim 12,wherein the semiconductor material is a semiconductor body, wherein adoped portion is embedded in the semiconductor body, wherein the dopedportion has a conductivity type opposite to a conductivity type of thesemiconductor material, wherein the doped portion is in contact with theelectrically conductive contact layer and with the semiconductormaterial, and wherein the doped portion has a conductivity type oppositeto the conductivity type of the semiconductor material.
 14. The methodof claim 1, wherein the nitrogen content of the electrically conductivecontact layer is between 38 to 45 atomic %.
 15. A method ofmanufacturing a semiconductor device, the method comprising: providing asemiconductor material that has a bandgap in between at least 2 eV andat most 10 eV and comprises SiC; and forming an electrically conductivecontact layer that comprises a metal nitride on the semiconductormaterial, wherein a non-ohmic contact is formed between thesemiconductor material and the electrically conductive contact layer,wherein forming the electrically conductive contact layer comprisesselecting the composition ratio of the metal nitride, wherein acomposition ratio is selected such that a nitrogen content of the metalnitride is between 10 and 50 atomic %.
 16. The method of claim 15,wherein the nitrogen content of the metal nitride is between 38 to 45atomic %.
 17. The method of claim 15, wherein the metal nitride directlycontacts the semiconductor material.
 18. A method of manufacturing asemiconductor device, the method comprising: forming a contact layer incontact with a semiconductor material, the semiconductor material havinga bandgap larger than 2 eV and less than 10 eV and comprising SiC, thecontact layer comprising a metal nitride, a nitrogen content of themetal nitride being between 38 and 45 atomic %; and forming a non-ohmiccontact between the semiconductor material and the contact layer. 19.The method of claim 18, wherein the metal nitride directly contacts thesemiconductor material.